The present invention relates to a vehicle suspension apparatus having a vehicle height control function capable of setting a vehicle height to at least a normal vehicle height and a high vehicle height, and higher than the normal vehicle height for automatically setting the vehicle height to the high vehicle height when travel on a rough road is detected.
A conventional suspension apparatus is known for increasing a vehicle clearance by raising the minimum ground height of a vehicle when traveling on a rough road to improve the drivability of the rough road by reducing contact of a vehicle body with the surface of the road, thereby reducing damage to the vehicle body. It utilizes a method of detecting a rough road when a counted value exceeds a set value by counting the number of reciprocations of a vehicle height sensor from a vehicle height higher than a set height range to another vehicle height lower than the set height range within a predetermined set time (FIG. 1). However, according to this conventional method, the center of the amplitude of the vehicle height value detected by the sensor becomes higher as shown in FIG. 2 after the height is controlled to a higher value when detecting a rough road. Thus, while the vehicle travels on a rough road, the vehicle height lower than the set height range cannot be obtained. Therefore, when the vehicle continuously travels on a rough road, the vehicle target height becomes "high" and "normal", resulting in a hunting of the vehicle height.
On the other hand, the vertical acceleration of the vehicle body can be detected by a G sensor to determine from the output of the sensor when a rough road is being traveled. However, when the output of the G sensor is alone used to detect whether or not the vehicle is running on a rough road, a delay can occur in determining a rough road condition due to a difference between the time the vehicle enters a rough road from a smooth road and the time the vehicle body is vertically accelerated, the latter occurring later than the former.